Preheat your oven to 350°F (175°C).
Line an 8x8-inch baking pan with parchment paper, leaving some overhang.
In a medium saucepan over low heat, combine the butter and chopped chocolate. Stir until melted and smooth, then let cool slightly.
In a large mixing bowl, whisk together the eggs, sugar, and vanilla extract. Pour in the melted chocolate mixture and whisk until smooth.
Sift in the cocoa powder and flour, folding gently with a spatula until just combined.
If using, fold in the toasted pecans or walnuts.
Pour half of the brownie batter into the prepared pan. Dollop the dulce de leche on top and spread slightly. Pour the remaining brownie batter over.
Bake for 25-30 minutes, or until a toothpick comes out with a few moist crumbs.
Let the brownies cool in the pan for about 15 minutes, then lift out using the parchment and cool completely before cutting.
